The present invention relates to a production process of a rubber composition and the rubber composition obtained by the process, and a tire using the rubber composition.
Mica used as filler in a rubber composition for tire is flat, superior in air-blocking property and shape-retaining property during process and preferable for use as the inner liner member of a tire.
If the air-blocking property of a rubber composition is improved, the butyl rubber amount in rubber components included in the rubber composition can be reduced (namely, a natural rubber (NR), an epoxidized natural rubber (ENR) or a butadiene rubber (BR) can be partially used as a rubber component), and LRR (low rolling resistance=low tan δ) is enabled because the heat build-up property of the whole rubber matrix is improved.
If mica is compounded as filler in a rubber composition for tire, rubber polymer component can be reduced, the compounding amounts of carbon black and silica can be reduced, and when the proportion of rubber polymer component is large in the rubber composition, the production cost of the rubber composition for tire is heightened, and the flatness of vulcanized rubber sheet, shrinkage after extrusion, sharp edge property and tensile property at high temperature tend to be lowered.
The total compounding amount of filler including mica in the rubber composition for tire has been conventionally preferably 45 to 70 parts by mass based on 100 parts by mass of the rubber components. Further, the compounding amount of mica in filler is preferably 15 to 45 parts by mass.
However, problems have been also generated by compounding mica as filler in the rubber composition for tire.
Mica has usually particle size of 25 to 100 μm. When the hardness of rubber compounding article produced is great, mica becomes easily destruction nuclei when filler dispersion is bad; therefore since crack growth property is accelerated, result not preferable for the rubber composition for tire is generated. As deduced from the fact that mica does not affect the viscosity of the rubber compounding article, it does not generate kneading shear (torque at mixing) and as a result, it causes the rotor slip (bad kneading) of Banbury mixer. In particular, when ENR having low rubber polymer viscosity and chlorobutyl HT 1066 with low viscosity are used, the dispersion of filler is bad and crack growth is also bad (namely, crack growth property is accelerated) because of the influence of low filler. For example, air permeability, moisture permeability and LRR can be expected to be enhanced in good balance by a rubber compounding system in which NR used in combination with butyl rubber as the rubber component used in the rubber composition for tire is exchanged with ENR, and ENR 25 and the butyl rubber are mixed at a mass ratio of 45:55. However, although ENR is wanted to be used, crack growth is bad because the viscosity of ENR is lower than that of NR as raw material, the viscosity of compounding is lowered entirely and filler is hardly separated; therefore it cannot be adopted.
Various procedures are presented for improving these problems. Various procedures and problems thereof are described below.
(1) The portion of butyl rubber and NR is replaced with BR. However, the air leak of a tire tends to increase. The order that air retention property is good is butyl rubber≈ENR 50>>ENR 25>NR>BR.
(2) Mica is not used in the rubber composition for tire as filler but flat clay with smaller particle diameter is used. However, the air leak of a tire tends to increase.
(3) ENR with high viscosity is prepared to be used as a rubber component. However, since a step of epoxidizing a natural rubber (TSR) is required, higher viscosity than TSR is substantially impossible. Accordingly, essential improvement is not attained.
(4) The compounding amount of filler compounded in the rubber composition for tire, for example, the compounding amount of silica is increased. This increases shear and improve dispersion, but it tends to run contrary to LRR (low rolling resistance=low tan δ).
(5) The crosslinking density of rubber included in the rubber composition for tire is reduced. In order to do so, there is a method of reducing the compounding amounts of sulfur used as a vulcanizing agent and a vulcanization accelerator and not using a vulcanization accelerating aid including sulfur (for example, TACKIROL V200 (alkylphenol-sulfur chloride condensate) available from Taoka Chemical Co., Ltd.). However, tan δ tends to be deteriorated (namely, LRR (low rolling resistance=low tan δ) is not attained).
For example, in patent literature 1, there is disclosed a rubber composition for inner liner including 10 to 50 parts by weight of mica having an aspect ratio of at least 50 and average particle diameter of 40 to 100 μm, based on 100 parts by weight of rubber components comprising 30 to 60% by mass of a butyl rubber, 20 to 50% by mass of a natural rubber or an isoprene rubber and 10 to 40% by mass of a butadiene rubber, in order to provide a rubber composition for inner liner that improves the air retention property of a tire, lowers hysteresis loss and further prevents crack growth. However, there is room for improvement that mica is prepared as a lubricant during kneading a rubber and inhibits the dispersion of polymer and carbon/silica, tan δ is low and elongation at break is low (wherein, because of including BR, crack property is improved).
[Patent literature 1] Japanese Unexamined Patent Publication No. 2006-328193